A K-1 is intended for those who intend to marry. It seems more visits and time together would be the best route. She can certainly apply for another B2, but the consulate officer will evaluate her merits. By law, all B2 applicants are already assumed to have the immigrant intent. It is her responsibility to overcome that obstacle and convince the consulate officer that she intends to return to home country after her authorized visit. That might be difficult. Good luck.